Encephalitis (en-sef-uh-LIE-tis) is inflammation of the brain. Viral infections are the most common cause of the condition.

Encephalitis can cause flu-like symptoms, such as a fever or severe headache. It can also cause confused thinking, seizures, or problems with senses or movement. However, many cases of encephalitis result in only mild flu-like symptoms or even no symptoms.

Severe cases of encephalitis, while relatively rare, can be life-threatening. Because the course of any single case of encephalitis can be unpredictable, it's important to get a timely diagnosis and treatment.

Most people with viral encephalitis have either no symptoms or mild flu-like symptoms, such as the following:

Headache

Fever

Aches in muscles or joints

Fatigue or weakness

More-serious cases require prompt medical care. Additional signs and symptoms of more serious encephalitis may include the following:

Confusion, agitation or hallucinations

Seizures

Loss of sensation or paralysis in certain areas of the face or body

Muscle weakness

Double vision

Perception of foul smells, such as burned meat or rotten eggs

Problems with speech or hearing

Loss of consciousness

Signs and symptoms in infants and young children may also include:

Bulging in the soft spots (fontanels) of the skull in infants

Nausea and vomiting

Body stiffness

Inconsolable crying

Poor feeding or not waking for a feeding

Irritability

When to see a doctor

Get immediate care if you or someone you know is experiencing any of the symptoms associated with more-severe cases of encephalitis. Severe headache, fever and altered consciousness require urgent care.

Infants and young children with any signs or symptoms of encephalitis should receive urgent care.

The exact cause of encephalitis is often unknown, but the most commonly diagnosed cause is a viral infection. Bacterial infections and noninfectious inflammatory conditions also may cause encephalitis.

An infection may result in one of two conditions affecting the brain:

Primary encephalitis occurs when a virus or other infectious agent directly infects the brain. The infection may be concentrated in one area or widespread. A primary infection may be a reactivation of a virus that had been inactive (latent) after a previous illness.

Secondary (postinfectious) encephalitis is a faulty immune system reaction in response to an infection elsewhere in the body. Instead of solely attacking the cells causing an infection, the immune system also mistakenly attacks healthy cells in the brain.

Secondary encephalitis often occurs two to three weeks after the initial infection. Rarely, secondary encephalitis occurs as a complication of a live virus vaccination.

Common viral causes

Common causes of encephalitis include:

Herpes simplex virus. There are two types of herpes simplex virus (HSV). Either type can cause encephalitis. HSV type 1 (HSV-1) is usually responsible for cold sores or fever blisters around your mouth, and HSV type 2 (HSV-2) commonly causes genital herpes. Encephalitis caused by HSV-1 is rare, but it has the potential to cause significant brain damage or death.

Other herpes viruses. Other herpes viruses that may cause encephalitis include the Epstein-Barr virus, which commonly causes infectious mononucleosis, and the varicella-zoster virus, which commonly causes chickenpox and shingles.

Enteroviruses. These viruses include the poliovirus and the coxsackievirus, which usually cause an illness with flu-like symptoms, eye inflammation and abdominal pain.

Mosquito-borne viruses. Arboviruses, or arthropod-borne viruses, are transmitted by mosquitoes or other blood-sucking insects. Mosquito-borne viruses can cause infections that include West Nile, La Crosse, St. Louis, western equine and eastern equine encephalitis.

Mosquitoes transfer the virus from a nonhuman host — such as a bird, chipmunk or horse — to humans. Symptoms of an infection may appear within a few days to a couple of weeks after exposure to an arbovirus.

Tick-borne viruses. The Powassan virus is a well-known tick-transmitted virus that causes encephalitis in the U.S. and Canada. Symptoms usually appear about a week after exposure to the virus.

Rabies virus. Infection with the rabies virus, which is usually transmitted by a bite from an infected animal, causes a rapid progression to encephalitis once symptoms begin. Rabies is a rare cause of encephalitis in the U.S.

Childhood infections. Common childhood infections — such as measles (rubeola), mumps and German measles (rubella) — used to be fairly common causes of secondary encephalitis. These causes are now rare because of the availability of vaccinations for these diseases.

Anyone can develop encephalitis. Factors that may increase the risk of the condition include:

Age. Some types of encephalitis are more prevalent or more severe in certain age groups. In general, young children and older adults are at greater risk of most types of viral encephalitis. Encephalitis from the herpes simplex virus tends to be more common in people 20 to 40 years of age.

Weakened immune system. People who have HIV/AIDS, take immune-suppressing drugs, or have another condition causing a compromised or weakened immune system are at increased risk of encephalitis.

Geographic regions. Mosquito-borne or tick-borne viruses are common in particular geographic regions.

Season of the year. Mosquito- and tick-borne diseases tend to be more prevalent in spring, summer and early fall in many areas of the United States. In warmer areas of the U.S., however, mosquitoes and ticks may be present year-round.

The complications resulting from encephalitis depend on several factors, including age, the cause of the infection, the severity of the initial illness and the time from disease onset to treatment.

In most cases, people with relatively mild illness recover within a few weeks with no long-term complications.

Complications of severe illness

Injury to the brain from inflammation can result in a number of problems. The most severe cases can result in coma or death.

Other complications — varying greatly in severity — may persist for months or be permanent:

Persistent fatigue

Weakness or lack of muscle coordination

Personality changes

Memory problems

Paralysis

Hearing or vision defects

Speech impairments

Because the onset of serious illness associated with encephalitis is usually severe and relatively sudden, seek emergency care. The emergency care team will likely include a specialist in disorders of the nervous system (neurologist).

Questions from your doctor

You may need to answer some of the following questions or answer questions on behalf of your child or another person with severe illness:

When did the symptoms begin?

Have you recently started taking any new medications? If so, what is the drug?

Have you been bitten by a mosquito or tick during the past few weeks?

Have you traveled recently? Where?

Have you recently had a cold, flu or other illness?

Are you up to date on your immunizations? When was your last one?

Have you had any exposure to wild animals or known toxins recently?

Have you had unprotected sex with a new or long-term sexual partner?

Do you have a condition or take any drugs that result in a weakened immune system?

Questions about symptoms, risk factors and medical history are important in making a diagnosis of encephalitis. Diagnostic tests that may be needed include the following:

Brain imaging. Brain imaging is often the first test if symptoms and patient history suggest the possibility of encephalitis. The images may reveal swelling of the brain or another condition that may be causing the symptoms, such as a tumor.

Technologies may include magnetic resonance imaging (MRI), which can produce detailed cross-sectional and 3-D images of the brain, or computerized tomography (CT), which produces cross-sectional images.

Spinal tap (lumbar puncture). With a spinal tap, the doctor inserts a needle into the lower back to extract cerebrospinal fluid (CSF), the protective fluid that surrounds the brain and spinal column. Changes in this fluid can indicate infection and inflammation in the brain.

In some cases, samples of CSF can be tested to identify the virus or other infectious agent.

Other lab tests. Samples of blood or urine, or of excretions from the back of the throat can be tested for viruses or other infectious agents.

Electroencephalogram (EEG). Your doctor may order an EEG, a test in which a series of electrodes are affixed to the scalp. The EEG records the electrical activity of the brain. Certain abnormal patterns in this activity may be consistent with a diagnosis of encephalitis.

Brain biopsy. Rarely, a procedure to remove a small sample of brain tissue (brain biopsy) is used if symptoms are worsening and treatments are having no effect.

Treatment for mild cases, which may be mistaken as the flu, mainly consists of:

Antiviral drugs

Cases of encephalitis due to certain viruses usually require intravenous antiviral treatments. Antiviral drugs commonly used to treat encephalitis include:

Acyclovir (Zovirax)

Ganciclovir (Cytovene)

Foscarnet (Foscavir)

Some viruses, such as insect-borne viruses, don't respond to these treatments. However, because the specific virus may not be identified immediately or at all, treatment with acyclovir is often begun immediately. This drug can be effective against the herpes simplex virus, which can result in significant complications, such as encephalitis, or death when not treated promptly.

Side effects of antiviral drugs may include nausea, vomiting, diarrhea, and muscle or joint soreness or pain. Rare serious problems may include abnormalities in kidney or liver function or suppression of bone marrow activity. Appropriate tests are used to monitor for serious adverse effects.

Supportive care

Additional supportive care also is needed in the hospital for people with severe encephalitis. The care may include:

Breathing assistance, as well as careful monitoring of breathing and heart function

Occupational therapy to develop everyday skills and to use adaptive products that help with everyday activities

Speech therapy to relearn muscle control and coordination to produce speech

Psychotherapy to learn coping strategies and new behavioral skills to improve mood disorders or address personality changes — with medication management if necessary

The best way to prevent viral encephalitis is to take precautions to avoid exposure to viruses that can cause the disease:

Practice good hygiene. Wash hands frequently and thoroughly with soap and water, particularly after using the toilet and before and after meals.

Don't share utensils. Don't share tableware and beverages.

Teach your children good habits. Teach your children to practice good hygiene and to avoid sharing utensils at home and school.

Get vaccinations. Keep your own and your children's vaccinations current. Before traveling, talk to your doctor about recommended vaccinations for different destinations.

Protection against mosquitoes and ticks

To minimize your exposure to mosquitoes and ticks, follow these tips:

Dress to protect yourself. Wear long-sleeved shirts and long pants if you're outside between dusk and dawn when mosquitoes are most active and when you're in a wooded area with tall grasses and shrubs where ticks are more common.

Lemon eucalyptus isn't recommended for use on children younger than 3 years old.

Mosquito repellents can be applied to both the skin and clothes. To apply repellent to your face, spray it on your hands and then wipe it on your face. If you're using both sunscreen and a repellent, apply sunscreen first.

Use EPA-recommended insecticide. The EPA also recommends the use of products containing permethrin. These products, which repel and kill tics and mosquitoes, are sprayed on clothing, tents and other outdoor gear. Permethrin shouldn't be applied to the skin.

Avoid mosquitoes. Refrain from unnecessary activity in places where mosquitoes are most prevalent. If possible, avoid being outdoors from dusk till dawn, when mosquitoes are most active.

Keep mosquitoes out of your home. Repair holes in window and door screens.

Get rid of water sources outside your home. Where possible, eliminate standing water in your yard, where mosquitoes can lay their eggs. Common problems include flowerpots or other gardening containers, flat roofs, old tires and clogged gutters.

Control mosquitoes in standing water. Fill ornamental pools with mosquito-eating fish. Use mosquito dunks — products that are toxic to mosquito larvae — in birdbaths, ponds and garden water barrels.

Look for outdoor signs of viral disease. If you notice sick or dying birds or animals, report your observations to your local health department.

Protection for young children

Insect repellents aren't recommended for use on infants younger than 2 months of age. Instead, cover an infant carrier or stroller with mosquito netting.

Tips for using mosquito repellent with children include the following:

Always assist children with the use of mosquito repellent.

Spray on clothing and exposed skin.

Apply the repellent when outdoors to lessen the risk of inhaling the repellent.

Spray repellent on your hands and then apply it to your child's face. Take care around the eyes and ears.

Don't use repellent on the hands of young children who may put their hands in their mouths.

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